The invention relates to a circuit arrangement for determining the radiation power of radiation which is incident on a photodiode, applies a voltage signal to a cathode of the reverse-biased photodiode and detects the photodiode voltage which is dropped across the photodiode.
A circuit arrangement such as this is known from TIETZE, Ulrich; SCHENK, Christoph: Halbleiterschaltungstechnik, [Semiconductor circuit technology], 9th Edition, Springer-Verlag, Berlin, 1989, pages 107–108. The known circuit arrangement has an operational amplifier, whose inverting input is connected to the cathode of the photodiode. The non-inverting input of the operational amplifier and the anode of the photodiode are connected to ground. The output of the operational amplifier is fed back via a non-reactive feedback resistance to the inverting input of the operational amplifier.
When light strikes the photodiode, a photocurrent flows via the photodiode and the feedback resistance, with the output of the operational amplifier being set to a voltage value which corresponds to the voltage drop across the feedback resistance caused by the photocurrent. The output voltage from the operational amplifier is then a measure of the photocurrent, and is thus a measure of the radiation power of the light striking the photodiode.
One disadvantage of the known circuit arrangement is its limited dynamic range. The radiation intensity of sunlight, in particular, varies to such a major extent that the photocurrent must be detected over a measurement range of 1:105. The known circuit arrangement is therefore suitable only to a limited extent for applications in which the aim is to detect the photocurrent from photodiodes installed in solar sensors.